In the field emission cathode, the electrons are emitted by applying a negative potential to a needle-like tip and applying a positive potential to an opposing anode. In this case, a field emission electron micrograph can be obtained if a fluorescent screen is employed for the anode. The field emission electron micrograph usually exhibits a geometrical pattern which reflects crystallographical regularity of a metal which forms the tip. If defined in terms of an emission angle, the micrograph appears in a region of about 1 rad. as viewed from the tip.
When the field emission cathode is put into practical use, however, only a portion of the above-mentioned wide emission angle is utilized. The emission angle will be explained below with reference to a schematic diagram of FIG. 1 which shows an electron optical system for electron beam convergence in an electron gun which employs the field emission cathode. A needle-like field emission cathode tip 1 which is welded to the center of a hair-pin filament 2 is impressed with a voltage of power supply 5 which is negative with respect to a first anode 3, and electrons are emitted from the tip of the cathode 1 due to the field emission. In this case, the emitted electrons spread to about 1 rad. in terms of emission angle as mentioned above. The electron ray 16 which has passed through an aperture 15 of the first anode 3 is converged by the effect of electrostatic lens which is produced by a potential difference across the first anode 3 and a second anode 4 that are connected to a power supply 6, and offers a fine spot of electron beam on a suitable convergence plane 17. It is possible to obtain more fine spot of electron beam by repeating the convergence by combining magnetic lenses. Here, the emitted electrons which can be utilized as an electron probe 16 is confined by the aperture of the first anode 3 because of the reasons mentioned below. Namely, an electron optical lens has aberrations which cannot be corrected, irrespective of whether it may be an electrostatic lens or a magnetic lens. Among them, a spherical aberration occupies a majority portion. Since the quantity of spherical aberration is so great, the electron beam 16 being utilized is limited to the vicinity of an optical axis 18. Further, if a spherical abberation coefficient is denoted by Cs and an aperture angle of the electron beam 16 by .alpha., the aberration is given by Cs.alpha..sup.3. Therefore, to obtain a fine electron beam 16 minimizing the aberration, the aperture angle .alpha. must be restricted to a small region. In a practical apparatus, the aperture angle .alpha. is about .alpha..ltorsim.10.sup.-3 rad. If the current density distribution of the first anode 3 is assumed to be uniform, the ratio of a solid angle (lsr) of the total electron emission to a solid angle (.pi..alpha..sup.2) of the electron beam 16 which passes through the aperture 15 becomes equal to a ratio of the total current in the field emission to the current of the fine electron beam 16. In practice, however, the current density is not uniform in the first anode 3 due to the crystallographical regularity. Further, the axial azimuth of the tip 1 is so selected that a current density of field emission electron micrograph becomes great at the central portion. In the above-mentioned case, therefore, the ratio of the whole current to the current being utilized will be about 1000 to 1.
With the practical apparatus, on the other hand, it is required to converge the electron ray as finely as possible and to draw a current (hereinafter referred to as probe current) as greatly as possible. For example, to obtain a probe current of the order of 0.1 .mu.A, the total emission current of the order of 1 mA is necessary.
Under a constant vacuum pressure, on the other hand, the field emission current becomes stable with the decrease in the current. The current fluctuation increases with the increase in the current; i.e., the field emission current becomes unstable. Further, when a predetermined current is drawn, the current becomes stable when vacuum pressure is lower. Therefore, even when it is attempted to obtain a great total emission current, the current fluctuation becomes so great that the apparatus becomes unusable. In practice, even when the vacuum pressure in an ordinary vacuum chamber is about 5.times.10.sup.-10 Torr, it is extremely difficult to stably draw the field emission current of 100 .mu.A for extended periods of time. Accordingly, it is difficult to obtain larger probe current.